Hepatic hemosiderosis is a commonly recognized finding in marmosets as well as other new world monkeys kept in zoologic, breeding and research colonies. Based on the absence of associated liver pathology or elevation in serum liver enzymes in affected animals, the condition has been considered to be of no clinical or pathologic significance. A high incidence of hepatic and systemic hemosiderosis was detected in marmosets from NIH's colony, who were submitted for routine necropsy to the Pathology unit, LSS, SSB, VRP, NCRR. These animals were used in behavior research and had all died or were euthanized due to spontaneous disease or trauma. The animals were fed a routine diet and were not given experimental drugs. Most marmosets were debilitated and cachectic at the time of death. Death was caused by a variety of conditions, with underlying, predisposing factors not always identified. The objective of this study is to determine the incidence and distribution of hemosiderosis in a marmoset (Callithrix jacchuis) colony maintained at NIH, judge its potential impact on animal health, and compare its pathogenesis and tissue distribution to human iron-overload conditions. In this retrospective study, necropsy records and microscopic tissue sections from 46 marmosets presented to the pathology unit for necropsy between 1989 and 1992 were examined. Marmosets were excluded from the study only if no archived tissues were available for review. Hemosiderosis was found in 95% of the marmosets studied. The significance of this may allow development of novel diagnostic and prophylactic regimes applicable to humans. The pattern is comparable to that found in human hereditary hemochromatosis and differes from that found in nutritional and transfusion-induced hemosiderosis.